Thermal shifts on a machine tool are, for example, the results of warping of components, e.g., frame components, wherein such warping may be caused by heating and in particular by inhomogeneously heating the components, for example, by heat-input from guiding or driving members mounted on one side. When a frame component is heated by guides and drives on one side, the material will expand on this heated side in accordance with the one-sided heating. Due to the one-sided heating the frame component will have a larger length on the heated side than on the opposite side whereby the component will warp. Such behavior leads to a position error which is to be compensated by compensating during the control of the machine tool and the axes of a machine tool, respectively.
Generally, the above-described deformations of components of the machine tool are referred to as thermal growth of the machine tool. Machine tools exhibit this thermal growth due to the coefficient of thermal expansion. Thermal growth results on the one hand from linear thermal expansion, for example, of a slide or a machine bed of the machine tool. This portion of the linear thermal expansion results from a homogeneous rise of temperature of the component multiplied by the coefficient of thermal expansion.
However, a second portion of the thermal growth results via inhomogeneous differences in temperature that may occur on components of the machine tool. The cause of such differences in temperature may, e.g., be an uneven heat-input into the components of the machine tool. For example, if on a component of the machine tool the drives and guides are mounted to the bottom, this bottom will be heated more strongly and faster than the top of the component, e.g., a slide of a linear axis of the machine tool. Thus, this frequently leads to the situation that a frame component of a machine tool has a heated or quickly heating surface on which guides and drives are placed, and a cooler or more slowly and less strongly heating surface. Such one-sided heating results to warping of the component exposed to such inhomogeneous heating.
Regarding the above-described thermally caused shifts on a machine tool, it is known in the prior art to reduce or avoid the thermally caused shifts by actively tempering the machine tool and the components thereof. Thus, it is possible to use a medium that is brought to a predetermined temperature or a temperature guided in accordance with a set value by means of a cooling unit for locally tempering some or all components of the machine tools, in particular for cooling, e.g., the centers of heat production on a machine tool, such as, for example, spindles or drives.
Here, such approach by means of actively tempering or cooling some or all parts of a machine tool may be used effectively for reducing or avoiding the described changes of length of components in case of homogeneous heating. However, due to the locally limited input of the cooling agent it is not possible to fully prevent the generation of differences in temperature or it is even intensified in part so that the thermally caused deformations, which can be attributed to warping of the frame components in consequence of differences in temperature on different sides of the components of the machine tool, cannot be fully prevented by actively tempering the machine tool or are even intensified.
In this case, it is known in the prior art to compensate thermally caused shifts on a machine tool by measuring one or more temperatures on components of the machine tool and by calculating a compensation value correlative to the measured temperature in machine control by superimposing the axis target position. Here, it is known in the prior art to perform a control compensation wherein compensation values are calculated in dependence of the measured temperature(s) or difference(s) in temperature on components of the machine tool.
This may be done, e.g., by the following approach by a formula:
                              Δ          ⁢                                          ⁢          A          ⁢                                          ⁢          1                =                                                            (                                                      T                                          REFERENCE_                      ⁢                      11                                                        -                                      T                                          BASE_                      ⁢                      11                                                                      )                            ·              K_                        ⁢            11                    +                                                    (                                                      T                                          REFERENCE_                      ⁢                      12                                                        -                                      T                                          BASE_                      ⁢                      12                                                                      )                            ·              K_                        ⁢            12                    +                                    …              ++                        ⁢                                          (                                                      T                                          REFERENCE_                      ⁢                      1                      ⁢                                                                                          ⁢                      N                                                        -                                      T                                          BASE_                      ⁢                      1                      ⁢                                                                                          ⁢                      N                                                                      )                            ·              K_                        ⁢            1            ⁢            N                                              (        1        )                                          Δ          ⁢                                          ⁢          AN                =                                                            (                                                      T                                          REFERENCE_N                      ⁢                                                                                          ⁢                      1                                                        -                                      T                                                                  BASE_                        ⁢                        N                                            ⁢                                                                                          ⁢                      1                                                                      )                            ·                              K_                ⁢                N                                      ⁢                                                  ⁢            1                    +                                                    (                                                      T                                                                  REFERENCE_                        ⁢                        N                                            ⁢                                                                                          ⁢                      2                                                        -                                      T                                                                  BASE_                        ⁢                        N                                            ⁢                                                                                          ⁢                      2                                                                      )                            ·                              K_                ⁢                N                                      ⁢                                                  ⁢            2                    +                                    …              ++                        ⁢                                          (                                                      T                                          REFERENCE_N                      ⁢                      N                                                        -                                      T                    BASE_NN                                                  )                            ·                              K_N                ⁢                N                                                                        (        2        )            
Here, ΔA1 is a correction value or compensation value for a first axis A1 of the machine tool, and ΔAN is a compensation value for an N-th axis AN of the machine tool. A reference temperature and a base temperature TREFERENCE—11 and TBASE—11 are detected on a component of the axis A1 of the machine tool and a corresponding difference in temperature is formed. Furthermore, a reference temperature and base temperature are each detected on other axes A1 to AN of the machine tool and a corresponding difference in temperature value is obtained.
In this course a compensation correction value is calculated for each of the axes, in particular the linear axes of the machine tool, which compensation correction value takes into consideration the differences in temperature, respectively multiplied by a compensation factor. K—11 to K—1N and K_N1 to K_NN are the respective compensation factors which are prefixed to the respective differences in temperature. These may be determined by simulation or experiments on the machine tool in order to be able to achieve a satisfying compensation result.
Here, it is preferred to superimpose or correct the target axis positions of the respective axes of the machine tool, which are predetermined in the machine control of an NC machine tool or CNC machine tool, by the calculated compensation correction values ΔA1 or ΔAN. For example, the target axis position of the axis A1 could be corrected by the calculated value ΔA1 to compensate the thermal shift in the direction of the axis A1.
Similar methods for compensating temperature-dependent changes of position on a machine tool are known, for example, from DE 198 00 033 A1 or DE 10 2004 044838 A1.
However, such methods for compensating temperature-dependent changes of position on a machine tool are imprecise. In addition, it has been tried for a long time to further reduce the primary and secondary processing times (and thus the costs per piece) of machining on a machine tool by increasing the dynamics of the machine axes so that as a result thereof the thermally caused deviations increase with each machine generation having increased dynamics. In particular, thermal shifts on a machine tool thus become more relevant from one generation of machine tools to the next because the described thermally caused shifts increase with the dynamics of the machine tool, particularly because the friction in the drive and guiding members and the heating resulting therefrom increase with acceleration and above all with the maximum speed.
Moreover, machine tools generally have a plurality of axes serially based upon each other. This means that the thermal shifts of the individual axes add up toward the tool or workpiece and the thermoelastic shifts that act on the tool or workpiece result in dependence of all axes serially based upon each other. This exponentiates the occurring position errors.
In particular, a deviation of position due to thermal warping or thermal deformations may not only occur in the advancing direction (as is in the case of pure longitudinal expansion) of a linear axis, but further a deviation of position may occur perpendicular to the advancing direction. Especially in machines having large projections, that is, travel distances, large thermal growths result from these described effects, which account for a large part of the inaccuracies remaining on the workpiece. Particularly, inhomogeneous heating and warping of components that may be traced back thereto in case of the same temperature rise input on one side lead to distinctly larger thermal shifts (especially in another spatial direction) than the linear thermal expansion in case of homogeneous heating.
The inventors in the present application have carried out measurements wherein it was found that though the proportions of the deviation at a tool tip are merely within a range of −0.15 to 0.3 per mil as standardized relative to the work distance of the machine, after all this represents about −100 to 150 μm for a work distance of 500 mm, wherein these values do not satisfy today's requirements of machining accuracy of workpieces on a machine tool and have to be reduced or compensated.